Gallium nitride (GaN), aluminum nitride (AlN), AlGaN, InGaN, AlGaInN and other group III nitride semiconductors have been attracting attention as materials for semiconductor light-emitting element that emit blue light or ultraviolet light. Blue laser diodes (blue LDs) are applied to high-density optical disks and displays, and blue light-emitting diodes (blue LEDs) are applied to displays, illuminations and the like. Furthermore, ultraviolet LDs are expected to be useful for biotechnology, and ultraviolet LEDs are expected to be a source of ultraviolet light in fluorescent lamps.
Group III nitride semiconductor substrates (e.g., GaN substrates) for LDs and LEDs usually are formed by vapor-phase epitaxial growth. Such group III nitride semiconductor substrates are formed by, for example, letting group III nitride semiconductor crystals heteroepitaxially grow on sapphire substrates by vapor-phase epitaxial growth. However, the lattice constants of a sapphire substrate and a GaN crystal differ by 13.8% and their linear expansion coefficients differ by 25.8%. Therefore, the crystallinity of a GaN thin film obtained through heteroepitaxial growth by vapor-phase epitaxial growth is not sufficient. Moreover, the dislocation density of crystals obtained by this method is usually 108 to 109 cm−2, and a decrease in dislocation density is an important task to be accomplished.
To accomplish this task, efforts have been made to reduce dislocation density, and for example, the ELOG (epitaxial lateral overgrowth) method has been developed. Although by using this method, dislocation density can be reduced to about 106 to about 107 cm−2, the production process is troublesome.
Moreover, a method has been proposed in which a GaN thick film of relatively low dislocation is formed on a support substrate such as sapphire, GaAs substrate or the like by vapor phase epitaxy (usually HYPE), and then the aforementioned support substrate is removed by etching or polishing. However, these methods cannot sufficiently reduce dislocation density and result in a structure in which dislocation is locally concentrated. Therefore, it is difficult to produce LDs and LEDs of high intensity and high reliability with such substrates.
Recently, it has been discovered that using an alkali metal or alkaline earth metal as a flux component, GaN, AlN and the like can be synthesized at relatively low temperatures/pressures of 750 to 1000° C. and a few dozen atmospheres. Since this method enables substrates that have relatively large areas and few defects to be produced easily, research has been actively carried out (e.g., Patent Documents 1 and 2).
Meanwhile, doped group III nitride semiconductor substrates generally are needed to produce a variety of devices (e.g., Non-Patent Document 1). When a group III nitride semiconductor substrate is obtained using an alkali metal or alkaline earth metal as a flux, it is necessary to control various dopants. For a semiconductor light-emitting element, an n-type substrate usually is needed. In the flux method also, some dopants have been investigated (e.g., Patent Document 3). Most examples of methods for producing an n-type substrate according to the flux method are methods in which Si mainly is used as a dopant and methods involving autodoping. On the other hand, in vapor phase epitaxy, in addition to Si, also Ge is known as an n-type dopant. However, it is known that when Ge is used for doping in a high concentration (for example, 1×1019 cm3) by vapor phase epitaxy, pit-like defects are generated on a crystalline thin film and enhancing the mobility is troublesome (Non-Patent Documents 1 and 2).
Moreover, when a high-intensity LED is considered as a semiconductor light-emitting element, since the light emitted from the active layer is emitted in all directions, an n-type substrate of low light absorption is required as a group III nitride semiconductor substrate for use as a substrate. Furthermore, to attain an element with high efficiency and high reliability in a high-intensity LED, it is necessary to reduce non-luminescent centers that do not contribute to light emission and deterioration of the element. For this, a group III nitride substrate having a controlled dislocation density is needed. Moreover, to introduce a carrier into an active layer efficiently, a conductive substrate having a controlled dopant concentration is needed as a substrate. To realize an LED of a higher intensity, it is a great task simultaneously to enhance both heat discharge from an element and efficiency of extracting light to outside of the element. In contrast, although there have been attempts in the past to achieve a group III nitride substrate of high transparency (Patent Document 4), a group III nitride substrate that is transparent and has a suitable carrier concentration and low dislocation and a light-emitting element using such a group III nitride substrate are not known.
Furthermore, with respect to LDs, light is extracted in the direction parallel to the principal surface of a group III nitride substrate. Therefore, light and electric current concentrate more on the substrate compared with LEDs, and it is thus particularly important that the dislocation density of a substrate is low and the dopant concentration thereof is controlled. In addition, in an LD using a group III nitride substrate, due to light escaping from the active layer into the group III nitride substrate side and due to the fact that light escaping onto the p-side electrode is reflected on the group III nitride substrate and as a result escapes to the group III nitride substrate side, there is a problem that a disruption is observed in the emission pattern (interference pattern and substrate propagation mode) of the LD (Patent Document 5).
Patent document 1: JP 2002-293696 A
Patent document 2: JP 2003-206198 A
Patent document 3: WO04/013385
Patent document 4: 2005-213075 A
Patent document 5: 2005-45239 A
Non-Patent Document 1: S. Nakamura et. al., Jpn. J. Appl. Phys. Vol. 31 (1992), pp. 2883-2888
Non-Patent Document 2: W. Gotz et. al., Materials Science and Engineering Vol. B59 (1999), pp. 211-217